The invention relates to electrical transformer circuits and related apparatus, and deals more particularly with novel improvements in the design of, and method of producing, radio frequency transformers, especially those which are employed to couple radio frequency energy from the output amplifiers of a radio transmitter to an antenna system.
Transformers are commonly employed in electrical circuitry for coupling electrical energy from one circuit to another, and are particularly selected in specific applications to perform transformations of voltage or current, as well as impedance or power matching functions between the coupled circuits. In designing such transformers for use in the circuitry of radio transmitters, key performance requirements include: especially compact size compatible with miniaturized electronic circuitry; minimization of flux leakage to avoid stray inductive coupling between adjacent circuits proximal to the transformer; high Q; and, high coupling efficiency over the broad radio frequency (RF) band of 2 to 30 megahertz.
The aforementioned performance requirements have been only partially satisfied by prior art transformer devices. For example, one known method of constructing a transformer of the so called "RF broad-band" type involves sleeving a number of hollow cylinders of ferrite material over the entire length of the brass tube. The ends of a number of such tube assemblies are then mounted parallel to each other between a pair of opposed, parallel plates of insulative material, the ends of each tube being disposed within apertures in the plates. The outer sides of each plate has a printed type circuit thereon which electrically connects selected ones of the tubes together when the ends of the tubes are soldered to adjacent printed circuit lands on the plates. An insulated conducting wire is then threaded through the interior of the open ended tubes to form "U" shaped windings. The tubes, interconnected by the printed circuitry, act as the "primary" of the transformer, while the insulated conducting wire threaded through the tubes, functions as the "secondary". A prior art transformer of the type described above is manufactured by the TRW Electronics Components Corporation and is identified in the manufacturer's applications note CT-113-71. While this prior art construction technique is suitable for some transformer applications, there are many instances when these transformers are rather inefficient in terms of their performance in relation to the physical space that they occupy, and in any event, these prior art transformers are relatively expensive to fabricate. As mentioned above, the ends of the brass tubes of the prior art construction are connected to the printed circuitry on the end plates; this connection is normally effected by soldering, however, in many high power amplifier applications, heat build-up may partially melt the solder to the extent that circuit performance is adversely affected and hence, in these particular applications the tubes must be joined by more expensive brazing processes which eliminate the melting problem.
Other known prior art includes: U.S. Pat. Nos. 3,717,808; 3,725,741; 3,961,292; French Pat. No. 1,288,166; and German Pat. No. 2,320,589. Each of these prior patents disclose transformer devices wherein the windings are coaxial in nature; coaxial transformer constructions, particularly in miniturized applications, are especially costly to manufacture since the longitudinal axes of the windings must be held in close coaxial alignment during assembly in order to produce a transformer which has satisfactory performance characteristics. More significantly however, none of the transformer designs disclosed in the above noted patents are suitable for use in miniturized radio circuitry since none of such designs would be capable of coupling high levels of energy (compared to their small relative size which would be necessitated by use in a miniturized radio circuit) without quickly overheating; heat build up within the windings of such previous transformer designs would of course, severly limit their power handling capacity. Moreover, it is often desireable, particularly in radio transmitter circuits, to mount the transformers with their longitudinal aspect closely adjacent the surface of a circuit board, in which case transformer designs of the type described in the mentioned patents (even if miniturized) would entrap substantial quantities of heat between the sides of the transformer and the circuit board thereby further reducing the power handling capacity of the transformer.
The present invention overcomes the deficiencies inherent in prior art designs by providing a transformer including a novel winding configuration which is especially effective in dissipating heat away from the transformer but yet is particularly compact in size in relation to the high power levels which it is capable of handling. According to the present invention, a single, unitary, straight length of tubing comprising an electrically conductive material such as copper or brass, is formed or "bent" into a configuration having a looped or bight portion and a pair of legs, the ends of such legs being disposed adjacent to each other. In the preferred form, the tubing is bent into a U-shaped member, whose legs are of essentially equal length, and a pair of cut-away portions providing openings are provided on each side of the looped portion adjacent the legs, there being a second set of cut-away portions providing a second set of openings respectively near the extremities of the legs. Hollow, cylindrical magnetic elements, such as ferrite or the like, are then sleeved over each leg of the tubing. An insulated conducting wire is twisted about itself as it is successively passed through the interior of the tubing a preselected number of times to form a number of coil turns which comprises one winding of the transformer, while the tubing itself is employed as a second winding. The ends of the conducting wire may be trained out through either the first or second set of openings to provide one set of winding taps for connection to one external circuit while the extremities of the tubing may be bent as desired and directly connected to a second external circuit thereby providing a second set of winding taps.
Accordingly, it is a primary object of the present invention to provide especially simple structure for a radio frequency transformer device which is highly economical to fabricate. As a corollary to the foregoing object, it is a further object to provide a novel method of constructing a transformer device which minimizes the labor needed to fabricate the same and eliminate several fabrication steps heretofore required in the production of prior art transformers of the character described above.
Another object of the invention is to provide a broadband transformer device of the type described, which completely obviates the need for solder joining the constituent portions of the transformer during fabrication thereof.
A still further object, related to the foregoing object, is to provide a broadband transformer which is highly reliable in operation and whose constituent parts are not vulnerable to the heating effect typically encountered in high power circuit applications over a broad range of radio frequencys.
Another object of the invention is to provide a broadband transformer which is physically compact in size and has superior flux coupling efficiency while using less magnetic loading material to enhance such coupling. Moreover, as a result of the invention's of superior flux coupling efficiency, the effective operating band-width of the transformer is improved.
A further significant object of the invention is to provide a broadband transformer of the type described which employs a tubular winding having a geometrical configuration which is particularly effective in dissipating heat entrapped therewithin.
A still further object of the invention is to provide a broadband transformer of the type described above whose longitudinal aspect may be mounted parallel, virtually flush with a specially adapted printed circuit board in a manner to maximize cooling of the transformer while also minimizing space requirements in the associated electronic circuit package.
Other a further objects of the invention will be made clear or become apparent in the course of the following description of preferred and alternate embodiments of the invention.